An OFDM (Orthogonal Frequency Division Multiplexing) system such as LTE (Long Term Evolution) standardized in the 3GPP (3rd Generation Partnership Project) described in Non-patent Document 1 or 2 attracts attention as a next generation communication system. In the OFDM, in order to notify a transmitter of a state of a communication channel, a CQI (Channel Quality Indicator) indicating reception quality in a receiver may be calculated to be fed back to the transmitter.
The transmitter side always controls, based on feedback information, optimal frequency allocation, modulation scheme, and coding rate. Reception characteristics can accordingly be maintained in a good state.
In Patent Document 1, there is proposed that a SNR (Signal to Noise power Ratio) is calculated on a communication capacity basis to be used for calculating a CQI, and also proposed a method for approximation by a log(SNR) in calculation of a communication capacity. However, in this method, there is no specific mention of detailed processing performed by the receiver.
Hereinafter, by taking the case of the LTE as an example, a well-known method for calculating a SNR for a wideband CQI by averaging SNRs of respective subbands in the entire band is described.
In the case of the LTE, there are mainly three pieces of information as feedback information from the receiver to the transmitter: namely, a CQI indicating reception quality in a band; a PMI (Precoding Matrix Indicator) indicating an optimal precoding matrix for the band; and a RI (Rank Indicator) indicating optimal rank information when information is transmitted from the transmitter to the receiver. The precoding means a technology for improving reception signal characteristics by performing, on the transmission side, linear processing suited to a channel state between transmission and reception beforehand.
In Non-patent Document 2 (3GPP TS36.213 v8.3.0), for transmission of feedback information by a PUSCH (Physical Uplink Shared Channel) and PUCCH (Physical Uplink Control Channel), there are defined report modes from respective receivers to transmitters as illustrated in FIG. 10 and FIG. 11. In other words, transverse directions in FIG. 10 and FIG. 11 indicate types of modes. The modes illustrated are described as, for example, 1-0, 1-1, 2-0, 2-1, 2-2, 3-0, and 3-1. On the other hand, longitudinal directions indicate types of feedback information. Illustrated feedback information indicated by a double circle indicates feedback information that must be reported. Hereinafter, each feedback information is described.
A wideband CQI (PMI) illustrated in each of FIGS. 10 and 11 indicates CQI (PMI) information common in the entire band. A subband CQI (PMI) illustrated in FIG. 10 indicates CQI (PMI) information in a certain subband. A best-M CQI illustrated in FIG. 10 indicates information obtained by extracting M better CQIs in order in the entire band. A UE selected subband CQI illustrated in FIG. 11 indicates CQI information in a subband selected by a UE (User Equipment). A best-M PMI illustrated in FIG. 10 indicates PMI information in a subband selected in a best-M CQI report. An index r illustrated in FIG. 10 indicates information of a band selected in a best-M. An index j illustrated in FIG. 11 indicates information of a band selected by the UE.
In general, when the wideband CQI is fed back from the receiver to the transmitter, if only a SNR for each subband is known, the SNRs of the respective subbands must be averaged in the entire band. Accurately calculating an average SNR by this method necessitates calculation of a root. The calculation of the root may become a bottleneck in signal processing. It is therefore desired to introduce an appropriate approximation method when the root is calculated.    Patent Document 1: Japanese Patent Publication No. 2008-526117    Non-patent Document 1: 3GPP, TS 36.211 v8.3.0 (6.3.4 Precoding) May, 2008    Non-patent Document 2: 3GPP, TS 36.213 v8.3.0 (7.2 UE Procedure for reporting channel quality indication (CQI), precoding matrix indicator (PMI) and rank indication (RI)) May, 2008    Non-patent Document 3: “Mathematics Introduction Course 8 for Science and Engineering Numerical Calculation”, by Ichiro Kawakami, Published by Iwanami Shoten, Publishers, April, 1989